1. Field of the Invention
The invention relates to a liquid crystal display and a method of fabricating the same, and more particularly to a liquid crystal display implementing in-plane switching where an electric field to be applied to liquid crystal is generated in a plane parallel to a substrate, and a method of fabricating the same.
2. Description of the Related Art
A conventional liquid crystal display having employed twisted nematic is accompanied with a problem of a narrow viewing angle. In order to solve this problem, there has been suggested an in-plane switching (IPS) type liquid crystal display in which an electric field to be applied to liquid crystal is generated in a plane parallel to a substrate.
An in-plane switching type liquid crystal display is suggested, for instance, in Japanese Unexamined Patent Publications Nos. 7-128683, 7-159786, and 8-220518.
FIG. 1 is a top plan view of an in-plane switching type liquid crystal display suggested in Japanese Unexamined Patent Publication No. 7-128683, FIG. 2 is a cross-sectional view taken along the line II--II in FIG. 1, and FIG. 3 is a cross-sectional view of the in-plane switching type liquid crystal display. Each of FIGS. 1 to 3 illustrates a pixel positioned at an intersection of an image signal line 861 and a scanning line 862.
As illustrated in FIGS. 1 to 3, the illustrated liquid crystal display is comprised of a substrate 810, a first comb-shaped electrode 801 formed on the substrate 810 and composed of an electrically conductive layer made of metal and so on, an interlayer insulating layer 811 covering the first comb-shaped electrode 801 and the substrate 810 therewith, a second comb-shaped electrode 802 formed on the interlayer insulating layer 811 and composed of an electrically conductive layer made of metal and so on, an alignment layer 820 formed over the second comb-shaped electrode 802 and the interlayer insulating film 811 for aligning liquid crystal, an opposing substrate 840 formed with a color layer (not illustrated), black matrix (not illustrated) and so on, an alignment layer 821 formed on a lower surface of the opposing substrate 840 for aligning liquid crystal, a liquid crystal layer 830 sandwiched between the alignment layers 820 and 821, and a thin film transistor (TFT) 863 for driving liquid crystal.
As is understood in view of FIGS. 2 or 3, the first and second comb-shaped layers 801 and 802 are electrically conductive layers formed in separate fabrication steps. The first and second comb-shaped layers 801 and 802 are separated from each other by the interlayer insulating film 811.
In operation, when the scanning line 862 is selected, a voltage on the image signal line 861 is transferred to the second comb-shaped electrode 802 through the thin film transistor 863. As a result, an electric field is generated between the first and second comb-shaped electrodes 801 and 802 in accordance with image data.
Liquid crystal molecules in the liquid crystal layer 830 are in advance aligned in a direction almost perpendicular to a plane of FIG. 3 by the alignment layers 820 and 821. These liquid crystal molecules are oriented in accordance with the electric field, as illustrated in FIG. 3. Thus, there is implemented in-plane switching.
FIGS. 4 and 5 illustrate an in-plane switching type liquid crystal display suggested in the above-mentioned Japanese Unexamined Patent Publication No. 7-128683. FIG. 4 is a top plane view of the liquid crystal display, and FIG. 5 is a cross-sectional view taken along the line V--V in FIG. 4.
In the illustrated liquid crystal display, a common line 903 is electrically connected to a first comb-shaped electrode 901 through a contact hole 904. Hence, the first comb-shaped electrode 901 and a second comb-shaped electrode 902 are formed in a common electrically conductive layer on a substrate 910. When a scanning line 962 is selected, a voltage on an image signal line 961 is transferred to a second comb-shaped electrode 902 through a thin film transistor 963. As a result, there is generated an electric field between the first and second electrodes 901 and 902. The thus generated electric field performs in-plane switching.
In accordance with the liquid crystal display suggested in the above-mentioned Publication, the first comb-shaped electrode 901 has the same height as that of the second comb-shaped electrode 902, as illustrated in FIG. 5, ensuring enhancement in degree of parallelization of an electric field performing in-plane switching.
The above-mentioned Japanese Unexamined Patent Publication No. 7159786 has suggested a liquid crystal display in which an alignment layer and an insulating film are designed to have a smaller dielectric constant than that of a liquid crystal layer to thereby generate an improved horizontal electric field, an insulating film and an alignment layer are composed of common material to thereby improve an efficiency of fabrication process, and electric fields for in-plane switching are made parallel.
The above-mentioned Japanese Unexamined Patent Publication No. 8220518 has suggested a liquid crystal display in which recessed and raised portions of a surface facing a liquid crystal layer are less formed to thereby ensure a higher contrast ratio.
However, with respect to display characteristics, an in-plane switching type liquid crystal display is accompanied with a problem of after-image, which is found also in a twisted nematic type liquid crystal display, but in a smaller degree. The problem of after-image in an in-plane switching type liquid crystal display is pointed out also in the above-mentioned Japanese Unexamined Patent Publication No. 7-157986.
It is said that such after-image is caused by electric charges residual at an alignment layer and/or at an interface between a substrate and a liquid crystal layer. It is considered that after-image is not generated, if liquid crystal is driven by an alternative current between first and second comb-shaped electrodes, and an absolute value of a positive voltage is substantially equal to an absolute value of a negative voltage in an alternative current.
However, the inventor of the present invention has found that flexo-electric effect found on a comb-shaped electrode has a close connection with generation of after-image.
In general, if bar-shaped liquid crystal molecules are radially aligned or spray-aligned, there would be generated polarization between inside and outside of radial alignment configuration. This is called flexo-electric effect. A degree of flexo-electric effect is dependent on a degree of extension of radial alignment configuration.
In the conventional liquid crystal display illustrated in FIGS. 1 to 3, whereas the interlayer insulating film 811 and the alignment layer 820 are sandwiched between the first comb-shaped electrode 801 and the liquid crystal layer 830, only the alignment layer 820 is sandwiched between the second comb-shaped electrode 802 and the liquid crystal layer 830, as illustrated in FIG. 3.
In other words, a dielectric distance between an upper surface of the first comb-shaped electrode 801 and the liquid crystal layer 830 is different from a dielectric distance between an upper surface of the second comb-shaped electrode 802 and the liquid crystal layer 830 by a thickness of the interlayer insulating film 811. Herein, a dielectric distance is defined as a capacitance per a unit area, obtained when sandwiched between imaginary electrodes.
As a result, a degree of extension in radial configuration of alignment of liquid crystal in the vicinity of the alignment layer 820 just above the first comb-shaped electrode 801 is different from the same in the vicinity of the alignment layer 820 just above the second comb-shaped electrode 802. Liquid crystal generates flexo-electric effect in dependence of the radial configuration, and polarize. It is considered that if the radial configuration of liquid crystal above the first comb-shaped electrode 801 is identical with the radial configuration of liquid crystal above the second comb-shaped electrode 802, polarization of liquid crystal is balanced, and if electric charges are accumulated on the first and second comb-shaped electrodes 801 and 802 to the same degree, a voltage based on a direct current is not residual across the first and second comb-shaped electrodes 801 and 802, even after in-plane switching has been stopped, which will not contribute to generation of after-image.
However, if the radial configuration of liquid crystal above the first comb-shaped electrode 801 has an extension in a different degree from an extension of the radial configuration of liquid crystal above the second comb-shaped electrode 802, polarization of liquid crystal is not balanced. As a result, electric charges are residual in implementing in-plane switching, which would cause a problem of after-image.
In the liquid crystal display illustrated in FIGS. 4 and 5, the first and second comb-shaped electrodes 901 and 902 are formed with no interlayer insulating layer being formed therebetween, and hence, can be formed in a common film-forming step. However, as illustrated in FIG. 6, in a photolithography step for forming an electrically conductive layer from which the first and second comb-shaped electrodes 901 and 902 are formed, a patterning defect 970 might occur because of etching residue or inaccuracy in photolithography. If the patterning defect 970 as illustrated in FIG. 6 occurs, the first comb-shaped electrode 901 would be readily short-circuited with the second comb-shaped electrode 902.
That is, the liquid crystal display illustrated in FIGS. 4 and 5 can advantageously suppress generation of after-image, but is accompanied with a problem of reduction in a fabrication yield, as mentioned above.